Walter E. Becker, in U.S. Pat. No. 3,228,741 (1966), discloses polysiloxane polymers for use in elastomeric contact lenses exhibiting exceptionally high oxygen permeability.
Charles S. Cleaver, in U.S. Pat. No. 3,981,798 (1976), discloses random copolymers produced from the copolymerization of perfluoroalkyl alkyl methacrylates or fluorine-containing telomer alcohol methacrylates and methylmethacrylate for use in rigid gas permeable contact lenses.
N. E. Gaylord, in U.S. Pat. No. 3,808,178 (1974) and Re 31,406 (1983), discloses random copolymers of polysiloxanyl acrylates and alkyl acrylic esters made by free radical polymerization for use in contact lenses.
N. E. Gaylord, in U.S. Pat. No. 4,120,570 (1978), describes a method for treating patients with visual defects by fitting them with rigid gas permeable contact lenses made from random copolymers of polysiloxanyl acrylates and alkyl acrylic esters.
N. E. Gaylord, in U.S. Pat. No. 3,808,179 (1974) discloses random copolymers produced from the copolymerization of fluoroacrylates and esters of acrylic acid and methacrylic acid for use in rigid gas permeable contact lenses.
K. A. Andrianov, et al., Bull. Acad. Sci. USSR Chem., No. 4, pp 467-472 (1957) describes the synthesis and polymerization of Organosilicone Compounds containing a methacrylol group.
R. L. Merker, et al., Journal of Organic Chemistry, 21, 1537 (1956) describes the synthesis and physical characteristics of copolymers of silicone acrylates and methyl methacrylate.
E. J. Ellis et al., in U.S. Pat. No. 4,152,508 (1979), discloses copolymers of siloxanyl alkyl acrylates, an itaconate ester, and an ester of acrylic or methacrylic acid. The copolymers preferably include a cross linking agent and a hydrophilic monomer.
E. R. Martin, in U.S. Pat. No. 3,878,263 (1975), discloses polysiloxane polymers obtained by equilibrating a mixture containing an acrylate-functional silane or siloxanes and a cyclic organopolysiloxane in the presence of a base catalyst and an aprotic solvent.
G. D. Friends et al., in U.S. Pat. No. 4,254,248 (1981), disclose copolymers prepared from monomeric polysiloxanes endcapped with activated double bonds and polycyclic esters of acrylic or methacrylic acid.
W. S. Covington, in U.S. Pat. No. 4,245,069 (1981), discloses copolymers and terpolymers of an addition cross-linked polysiloxane and one or more acrylic or methacrylic esters of certain hydroxy or alkoxy alcohols.
W. G. Deichert et al., in U.S. Pat. No. 4,189,546 (1980), disclose crosslinked networks prepared from a poly(organosiloxane) monomer alpha, omega-terminally bonded through divalent hydrocarbon groups to free radical-polymerizable unsaturated groups. Homopolymers and copolymers are disclosed.
A. Aoki et al., in U.S. Pat. No. 4,304,881 (1981), disclose the preparation of styrene/butadiene "living" polymers by anionic polymerization and coupling of these polymers by reaction with silicone tetrachloride to produce a 4-arm star polymer having a silicone atom as a core.
O. W. Webster, in U.S. Pat. Nos. 4,417,034 (1983) and 4,508,880 (1985) and W. B. Farnham and D. Y. Sogah in U.S. Pat. Nos. 4,414,372 (1983) and 4,524,196 (1985) disclose the preparation of acrylic star polymers using group transfer polymerization by coupling "living" polymer with a capping agent having more than one reactive site or by initiating polymerization with an initiator which can initiate more than one polymer chain. Initiators that could produce acrylic star polymers with up to 4 arms were demonstrated.
H. J. Spinelli, in U.S. Pat. Nos. 4,659,782 and 4,659,783 issued (1987), teaches the preparation of acrylic star polymers with crosslinked cores and at least 5 arms, optionally having functional groups in the cores and/or the arms. Group transfer polymerization being preferably used to make the polymers is disclosed.
As is true for most bio-medical applications, polymers that are to be used in contact lens applications have very demanding requirements placed on them. For example, rigid gas permeable contact lenses, like other contact lenses, not only must be hard and machineable, but also highly oxygen permeable. In addition, these lenses must be comfortable to wear. Furthermore, these contact lenses should have the characteristics of good flex resistance, adequate wettability and non-adherence to the eye. It is also important that the lenses maintain their shape after extended use. Finally, the lenses should be resistant to deposits of proteins, lipids, and bacteria.
In the case of soft contact lenses, these should be oxygen permeable, drapeable, wettable, durable, have adequate tear-strength, clarity and resistance to deposits of proteins, lipids and bacteria.
Initially, contact lenses were made from polymethylmethacrylate (PMMA), a hard, easily machineable polymer. These lenses were reasonably comfortable to wear but were not sufficiently permeable to oxygen. Consequently, they could be worn only for limited periods of time. Wearing such lenses for prolonged periods of time sometimes resulted in serious eye damage.
The next generation of lenses were the soft lenses made from polyhydroxyethylmethacrylate (PHEMA) containing high concentrations of water. These hydrogels transport more oxygen than does PMMA because the polymers accommodate large concentrations of water, but the lenses are difficult to manufacture and handle because of their softness. The increased oxygen transport is associated with the solubility of oxygen in water rather than to the polymer per se. In addition to being soft and deformable, the hydrogels from PHEMA are very susceptible to deposits and lack tear resistance.
The most recent generation of lenses, the rigid, oxygen-permeable lenses, are made from random copolymers of silicone acrylates and methylmethacrylates such as TRIS(trimethylsiloxy)-3-methacryloxypropylsilane (TRIS) and methyl methacrylate. These lenses have a significantly higher oxygen permeability than lenses made from either PMMA or PHEMA hydrogels. Lenses made from TRIS homopolymer have very high oxygen permeability but they are soft, lack wettability, do not resist deposits well, and are very uncomfortable to wear. Using TRIS copolymerized with methyl methacrylate increases the durability and machinability, but there is a trade-off in other properties, most notably the oxygen permeability. The manufacturer can provide lenses with high silicone content that can be worn for extended periods of time but are very difficult to make or harder lenses with relatively high methyl methacrylate content that are more easily machineable but have reduced oxygen permeability.
Other monomers that have been used in making contact lenses often improve one property at the expense of others. For example, hexafluorobutyl methacrylate (HFBMA) gives excellent resistance to deposits but is less oxygen permeable than are the silicone acrylates. Lenses made from dimethylsilicone elastomers have very high oxygen permeability but are very soft, and difficult to manufacture, extremely non-wettable, and very uncomfortable to wear.
One of the current processes for making materials for oxygen permeable contact lenses involves the bulk free radical copolymerization of an alkyl (meth)acrylate, for example methyl methacrylate, with a polysiloxanylalkyl ester of acrylate or methacrylate (silicone acrylate), among others, for example TRIS, and an amount of a polyfunctional monomer, such as ethyleneglycol dimethacrylate, to provide rigidity through crosslinking. As mentioned above, there results a trade-off in properties depending upon the relative proportions of the monomers used. It was originally believed that lens materials having high oxygen permeability and improved hardness and machinability could be made by incorporating a hard polymer such as PMMA in the bulk polymerization of a silicone acrylate with an alkyl acrylic ester. It has been found, however, that PMMA is not soluble in silicone acrylate monomers nor in their mixtures with alkyl acrylic esters; nor has it been possible to incorporate PMMA in the highly oxygen permeable dimethylsilicone elastomers.
Surprisingly, pre-formed macromonomers and star polymers of the present invention can be dissolved or dispersed in silicone acrylate monomers, wetting monomers such as hydroxyethylmethacrylate, glycerol methacrylate, polyvinyl alcohols, polyvinylpyrrolidone and methacrylic acid, among others, and/or mixtures of such monomers with alkyl acrylic esters. The copolymers of the present invention may be adapted for use in hard, flexible or soft contact lenses. It has been found that bulk polymerization of these mixtures gives products with attractive properties including optical clarity, suitable hardness (in the case of rigid gas permeable lenses), and enhanced oxygen permeability, for use in making hard, flexible and soft contact lenses.